Recently, based on the development of optical technology, display devices using various schemes replacing a Braun tube in the related art, such as a plasma display panel (PDP), a liquid crystal display (LCD), or the like have been proposed and have come into the market. The characteristics required for a polymer material used in these displays have been further enhanced. For example, in the case of an LCD, as thinning, weight reduction and increases in screen area have been undertaken, in particular, a widened viewing angle, a high contrast, the control of image color change due to an viewing angle, and a uniform screen display have become important.
Accordingly, various polymer films have been used for a polarizing film, a polarizer protective film, a retardation film, a plastic film, a light guiding plate, and like. Various modes of liquid crystal display using twisted nematic (TN), super twisted nematic (STN), vertical alignment (VA), and in-plane switching (IPS) liquid crystal cells, as liquid crystals, have been developed. Since all of the liquid crystal cells have a unique liquid crystal arrangement, they have unique optical anisotropy. In order to compensate for optical anisotropy, a film having a phase difference function provided by stretching various kinds of polymers has been proposed.
A polarizing plate generally has a structure in which a triacetyl cellulose film (hereinafter, referred to as TAC film) as a protective film is stacked on a polarizer by using water-based adhesive made of polyvinyl alcohol solution. However, neither a polyvinyl alcohol film used as a polarizer nor a TAC film used as a polarizer protective film have sufficient heat and humidity resistance properties. Accordingly, when a polarizing plate made of these films is used in a high temperature or high humidity atmosphere for an extended period of time, there are various limitations in terms of the uses thereof because polarization may be degraded, the polarizer may be separated from the protective film or optical properties may be deteriorated.
In addition, a TAC film has serious changes in an existing in-plane phase difference Rin and a thickness direction phase difference Rth according to ambient temperature/humidity, and in particular, has a significant change in phase difference with respect to incident light in a dip direction. When a polarizing plate including the TAC film having such characteristics as a protective film is applied to a liquid crystal display, viewing angle properties are varied according to changes in ambient temperature/humidity circumstances to thereby cause the degradation of image quality. Furthermore, since TAC film has a relatively large photoelastic coefficient value, as well as a large dimensional change rate according to the changes in ambient temperature/humidity circumstances, it may have local changes in phase difference properties generated after durability evaluation in heat-resistant, humidity-resistive circumstances, whereby image quality may tend to be deteriorated.
As a material compensating for several disadvantages of the TAC film, a methacrylate-based resin is well known. However, since the methacrylate-based resin may tend to be fragile and easily cracked, it is known that the methacrylate-based resin has limitations in terms of conveyance and productivity at the time of the manufacturing of a polarizing plate.
In order to solve these defects, methods of blending other resins or a tough conditioner into an acrylic resin (Japanese Patent Laid-Open Publication No. 2006-284881 and Japanese Patent Laid-Open Publication No. 2006-284882), or methods of co-extruding and stacking other resins (Japanese Patent Laid-Open Publication No. 2006-243681, Japanese Patent Laid-Open Publication No. 2006-215463, Japanese Patent Laid-Open Publication No. 2006-215465, and Japanese Patent Laid-Open Publication No. 2007-017555) have been proposed. However, these methods have limitations in that inherent high heat-resistance and high transparency of the acrylic resin are not sufficiently reflected therein or that the acrylic resin has a complex laminate structure.